To change the date format to dd/mm/yy - go to your Google Apps gear wheel -> settings -> Language -> click the drop down box and select English (UK) - that gets you dd/mm/yyyy by default. Sorry if you are anything but British, but that seems the best way to do it as a user. I'm trying to do it for a whole domain to save users using different date formats and getting snarled up - not found how to do it yet!

There is a setting in Google Admin control panel -> Domain settings -> Language, which I've set to English (UK), but I've noticed one of my users is still coming up showing mm/dd/yyyy format. Meh.